Hafnocene complexes reaction with ethylene

The actual reaction path followed becomes more evident if the corresponding hafnium complexes are used as substrates. In this case the crucial intermediate 29, which can react to form either product 30 or 17, can be isolated. The thermodynamically favored ( -m-butadiene)hafnocene (5b) (32) turns out to be inert toward ethylene under the conditions applied. Even heating to 120°C in an ethylene atmosphere (1 bar) for several hours does not result in consumption of the metal complex. In contrast, s-trans-rf-butadiene)hafnocene (3b) rapidly takes up 1 molar equivalent of C2H4 even at -10°C to yield a C—C coupling product, i.e., the five-membered metallacyclic cr-allylhafnocene complex 29b. Above 0°C, vinylhafnacy-clopentane reacts with additional ethylene to form bis(cyclo-pentadienyl)hafnacyclopentane (30b) and free butadiene. In the absence... 

The reaction of (EBI)H2 [EBI = ethylene-1,2-bis(indenyl)] and titanium amides did not give the expected ansa-titanocene products. Even with the azetidine complex, Ti(NC3H6)4 reacts with only a single amine elimination to give the mono(indenyl) derivative (C9H6CH2CH2C9H7)Ti(NC3H6)3. By contrast, -zirconocene and hafnocene are easily obtained by the amine elimination process. It is likely that increased steric crowding around the smaller Ti disfavors the second amine elimination.674... 

Alkene complexes of Ti, Zr and Hf have been intensively investigated with regard to the nature of bonding and the close relation to olefin oligomerization and polymerization. Alkene complexes of zirconocene and hafnocene are isolated as the trimethylphosphine adduct, Cp2M(T -alkene)(PMe3) (33) [92-94]. Cp 2Ti(CH2=CH2) (34) is a 16 electron ethylene complex with a rich reaction chemistry as summarized in Scheme 6.4 [95-99]. The reaction profile of 34 indicates that the metallacyclopropane canonical form makes an important contribution [100]. 

---